COMP3131/9102: Programming Languages and Compilers

Transcription

1 COMP3131/9102: Programming Languages and Compilers Jingling Xue School of Computer Science and Engineering The University of New South Wales Sydney, NSW 2052, Australia Jingling Xue COMP3131/9102 Page 513 May 8, 2017

2 Lectures on Java Byte Code Generation 1. Lecture 10: Code generation (Assignment 5) 2. Lecture 11: Chris lecture 3. Lecture 12: Code generation + Revision COMP3131/9102 Page 514 May 8, 2017

3 Understanding Jasmin Assembly Language 1. Read syntax.bnf to understand Jasmin s syntax 2. Read Jasmin User Guide to Jasmin s syntax 3. Read Jasmin instruction reference manual to understand its instructions (1-to-1 mapped to JVM instructions) 4. To under a particular feature, do the following: (a) Design a Java program Test.java (b) javac -g Test.java (c) javap -c Test > Test.j (jasmind Test.class) (d) Study the Jasmin code in Test.j COMP3131/9102 Page 515 May 8, 2017

4 1. Translation: Lecture 10: Java Byte Code Generation Expressions (including actual parameters) Statements Declarations (including formal parameters) 2. Allocating variable indices for local variables 3. Some special code generation issues: lvalue (store) v.s rvalue (load) assignment expressions such as a = b[1+i] = 1 Expression statements such as 1 + (a = 2); Short-circuit evaluations break and continue return 4. Generating Jasmin assembler directives.limit stack.limit locals.var.line COMP3131/9102 Page 516 May 8, 2017

5 VC.lang.System.class The VC Compiler Source Code (gcd.vc) Assignment 1: Scanner Tokens Assignment 2 & 3: Parser AST Assignment 4: Semantic Analyser Decorated AST Assignment 5: Code Generator Jasmin Assembly Code (gcd.j) Jasmin Assembler Java Byte Code (gcd.class) Java Virtual Machine Results COMP3131/9102 Page 517 May 8, 2017

6 Object Code Generation for Register-Based Machines Three issues: register allocation instruction selection code scheduling These are simpler for stack-based machines, especially if the quality of the generated code is not a concern Advanced Compiler Construction: Code Optimisations Object code generation for register-based machines Research-oriented topics COMP3131/9102 Page 518 May 8, 2017

7 Example 1: gcd.vc int i = 2; int j = 4; int gcd(int a, int b) { if (b == 0) return a; else return gcd(b, a - (a/b) *b); } int main() { putintln(gcd(i, j)); return 0; // optional in VC or C/C++ } COMP3131/9102 Page 519 May 8, 2017

8 Example 1: gcd.vc(red Assumed by the VC Compiler) public class gcd { static int i = 2; static int j = 4; public gcd() { } // the default constructor int gcd(int a, int b) { if (b == 0) return a; else return gcd(b, a - (a/b) *b); } } void main(string argv[]) { gcd vc$; vc$ = new gcd(); System.putIntLn(vc$.gcd(i, j)); return; } COMP3131/9102 Page 520 May 8, 2017

9 int main() is assumed to be: Example 1: gcd.vc (cont d) public static void main(string argv[]) {... } visitfuncdec: a return is always emitted just in case no return expr was present in the main of a VC program visitreturnstmt: emit a RETURN rather than IRETURN even if a return statement, e.g., return expr is present in the main of a VC program All VC functions are assumed to be instance methods with the package access All global variables are assumed to be static field variables with the package access All built-in VC functions are static COMP3131/9102 Page 521 May 8, 2017

10 .class public gcd.super java/lang/object.field static i I.field static j I Example 1: gcd.j ; standard class static initializer.method static <clinit>()v iconst_2 putstatic gcd/i I iconst_4 putstatic gcd/j I ; set limits used by this method.limit locals 0.limit stack 1 return.end method ; standard constructor initializer.method public <init>()v.limit stack 1.limit locals 1 aload_0 invokespecial java/lang/object/<init>()v return.end method.method gcd(ii)i L0:.var 0 is this Lgcd; from L0 to L1.var 1 is a I from L0 to L1 COMP3131/9102 Page 522 May 8, 2017

11 .var 2 is b I from L0 to L1 iload_2 iconst_0 if_icmpeq L5 iconst_0 goto L6 L5: iconst_1 L6: ifeq L3 iload_1 ireturn goto L4 L3: aload_0 iload_2 iload_1 iload_1 iload_2 idiv iload_2 imul isub invokevirtual gcd/gcd(ii)i ireturn L4: L1: nop ; set limits used by this method.limit locals 3.limit stack 5.end method.method public static main([ljava/lang/string;)v L0:.var 0 is argv [Ljava/lang/String; from L0 to L1.var 1 is vc$ Lgcd; from L0 to L1 new gcd dup COMP3131/9102 Page 523 May 8, 2017

12 L1: invokenonvirtual gcd/<init>()v astore_1 aload_1 getstatic gcd/i I getstatic gcd/j I invokevirtual gcd/gcd(ii)i invokestatic VC/lang/System/putIntLn(I)V ; The following return inserted by the VC compiler return ; set limits used by this method.limit locals 2.limit stack 3.end method COMP3131/9102 Page 524 May 8, 2017

13 The Translation of the gcd Method by the Java Compiler.method gcd(ii)i // more optimised!.limit stack 5.limit locals 3.var 0 is this Lgcd; from Label1 to Label2.var 1 is arg0 I from Label1 to Label2.var 2 is arg1 I from Label1 to Label2 Label1:.line 10 iload_2 ifne Label0.line 11 iload_1 ireturn Label0:.line 13 aload_0 iload_2 iload_1 iload_1 iload_2 idiv iload_2 imul isub invokevirtual gcd/gcd(ii)i Label2: ireturn.end method COMP3131/9102 Page 525 May 8, 2017

14 Code Generator as a Visitor Object Visitor (as an Object): implementing VC.ASTs.visitor Syntax-driven: traversing the AST to emit code in pre-, inor post-order or any of their combinations Classes: Emitter.java: the visitor class for generating code JVM.java: The class defining the simple JVM used Instruction.java: The class defining Jasmin instructions Frame.java: The class for info about labels, local variable indices, etc. for a function COMP3131/9102 Page 526 May 8, 2017

15 Code Template [[X]]: the code generated for construct X Code template: a specification of [[X]] in terms of the codes for its syntactic components A code template specifies the translation of a construct independently of the context in which it is used Compiled code always executes in some context Optimisation is the art of captalising on context! Lack of context fully general (i.e., slow) code Thus, inefficient code may be generated; it can be optimised later by the compiler backend COMP3131/9102 Page 527 May 8, 2017

16 Expressions 1. Literals 2. Variables (lvalues and rvalues) 3. Arithmetic expressions 4. Boolean expressions 5. Relational expressions 6. Assignment expressions 7. Call expressions (assignment spec) COMP3131/9102 Page 528 May 8, 2017

17 Integer Literals CodeTemplate: [[IntLiteral]]: emiticonst(intliteral.value) private void emiticonst(int value) { if (value == -1) emit(jvm.iconst_m1); else if (value >= 0 && value <= 5) emit(jvm.iconst + "_" + value); else if (value >= -128 && value <= 127) emit(jvm.bipush, value); else if (value >= && value <= 32767) emit(jvm.sipush, value); else emit(jvm.ldc, value); } Visitor method: public Object visitintliteral(intliteral ast, Object o) { Frame frame = (Frame) o; emiticonst(integer.parseint(ast.spelling));... return null; } COMP3131/9102 Page 529 May 8, 2017

18 Floating-Point Literals CodeTemplate: [[F loatliteral]]: emitfconst(floatliteral.value) private void emitfconst(float value) { if(value == 0.0) emit(jvm.fconst_0); else if(value == 1.0) emit(jvm.fconst_1); else if(value == 2.0) emit(jvm.fconst_2); else emit(jvm.ldc, value); } Visitor method: public Object visitfloatliteral(floatliteral ast, Object o) { Frame frame = (Frame) o; emitfconst(float.parsefloat(ast.spelling));... return null; } COMP3131/9102 Page 530 May 8, 2017

19 Boolean Literals CodeTemplate: [[BooleanLiteral]]: emitbconst(booleanliteral.value) private void emitfconst(boolean value) { if (value) emit(jvm.iconst_1); else emit(jvm.iconst_0); } Visitor method: public Object visitbooleanliteral(booleanliteral ast, Object o) { Frame frame = (Frame) o; emitbconst(ast.spelling.equals("true"));... return null; } COMP3131/9102 Page 531 May 8, 2017

20 Code template: Visitor Method: Arithmetic Expression E 1 i+ E 2 [[E 1 i+ E 2 ]]: [[E 1 ]] [[E 2 ]] emit( iadd ) public Object visitbinaryexpr(binaryexpr ast, Object o) { Frame frame = (Frame) o; String op = ast.o.spelling; ast.e1.visit(this, o); ast.e2.visit(this, o);... else if (op.equals("i+")) { emit(jvm.iadd);... }... Other arithmetic operators (integral or real) handled similarly COMP3131/9102 Page 532 May 8, 2017

21 AST: Example 1: ( ) The nodes visited in post-order per code template Code: iconst_1 bipush 100 iadd sipush 200 ldc iadd iadd COMP3131/9102 Page 533 May 8, 2017

22 visitfuncdecl: Frame Objects A new frame object created each time visitfuncdecl is called public Object visitfuncdecl(funcdecl ast, Object o) {... frame = new Frame(true) for main or new Frame(false) otherwise... The frame object passed as the 2nd arg and available at all child nodes The constructor of the class Frame: public Frame(boolean _main) { this._main = _main; label = 0; localvarindex = 0; currentstacksize = 0; maximumstacksize = 0; constack = new Stack<String>(); brkstack = new Stack<String>(); scopestart = new Stack<String>(); scopeend = new Stack<String>(); } Code will be provided COMP3131/9102 Page 534 May 8, 2017

23 L1: L2: Boolean (or Logical) Expressions: E 1 &&E 2 [[E 1 ]] ifeq L1 [[E 2 ]] ifeq L1 iconst 1 goto L2 iconst 0 public Object visitbinaryexpr(binaryexpr ast, Object o){ Frame frame = (Frame) o;... L1 = frame.getnewlabel(); L2 = frame.getnewlabel(); ast.e1.visit(this, o); emit(jvm.ifeq, L1); ast.e2.visit(this, o); emit(jvm.ifeq, L1); emit(jvm.iconst 1); emit(jvm.goto, L2); emit(l1 + : ); emit(jvm.iconst 0); emit(l2 + : );... Code must respect the short circuit evaluation rule and!dealt with similarly Better codes can be generated (Week 11 Tutorial) COMP3131/9102 Page 535 May 8, 2017

24 Example 2: Boolean Expressions: true && false iconst 1 ifeq L2 iconst 0 ifeq L2 iconst 1 goto L3 label=0 frame.getnewlabel called twice L2: iconst 0 L3: label=2 The Frame object created for main Passed to all the children of the main s FuncDecl node COMP3131/9102 Page 536 May 8, 2017

25 Example: Testing and Marking Short-Circuit Evaluation boolean f() { putbool(false); return false; } void main() { false && f(); } Wrong if false is printed! COMP3131/9102 Page 537 May 8, 2017

26 Code Template: Relational Expressions: E 1 i> E 2 L1: L2: [[E 1 ]] [[E 2 ]] if icmpgt L1 iconst 0 goto L2 iconst 1 Other relational operations on integer operands handled similarly COMP3131/9102 Page 538 May 8, 2017

27 AST: Example 3: Relational Expressions Code L0 and L1 generated in visitcompstmt L2: L3: iconst_1 iconst_2 iadd iconst_3 if_icmpgt L2 iconst_0 goto L3 iconst_1 COMP3131/9102 Page 539 May 8, 2017

28 Code Template: Relational Expressions: E 1 f> E 2 L1: L2: [[E 1 ]] [[E 2 ]] fcmpg ifgt L1 iconst 0 goto L2 iconst 1 if fcmpgt is non-existent and is simulated by fcmpg and ifgt Other floating-point relational operators handled similarly COMP3131/9102 Page 540 May 8, 2017

29 Assignment Expression: a = E Assumptions: (1) a is int (2) Its local variable index is 1 Code Template: [[E]] istore 1 The above code template breaks down for a = b = 1; iconst 1 dup istore 2 // the local var index for b is 2 istore 1 Need to know the context in which b = 1 is used when the node for b=1 is visited How? a parent link is added to every AST node ast.parent is not dup COMP3131/9102 Page 541 May 8, 2017

30 Code Template: Example: Assignment Expression: LHS = RHS [[LHS]] [[RHS]] appropriate store instruction VC: int[] a = new int[10]; // index 1 int i = 1; // index 2 int j = 2; // index 3 a[i + 1] = j + 10; Bytecode for a[i + 1] = j + 10: aload_1 iload_2 iconst_1 iadd iload_3 bipush 10 iadd iastore COMP3131/9102 Page 542 May 8, 2017

31 Statements 1. if 2. while for left for you to work it out 3. break and continue 4. return 5. expression statement 6. compound statement COMP3131/9102 Page 543 May 8, 2017

32 Code Template: if (E) S1 else S2 L1: L2: [[E]] ifeq L1 [[S1]] goto L2 [[S2]] Works even when either S1 or S2 or both are empty In the AST, if (E) S1 without the else is represented as IfStmt / \ E S1 EmptyStmt Those instructions in blue need not be generated. COMP3131/9102 Page 544 May 8, 2017

33 Code Template: L1: L2: while (E) S Push the continue label L1 to constack Push the break label L2 to brkstack [[E]] ifeq L2 [[S]] goto L1 Also works when S is empty Pop the continue label L1 from constack Pop the break label L2 from brkstack COMP3131/9102 Page 545 May 8, 2017

34 break and continue Code template for break: goto the label marking the inst following the while Code template for continue: goto the label marking the first inst of the while COMP3131/9102 Page 546 May 8, 2017

35 return E Assumption: type coercion has been done. Code Template: return E:int and return E:Boolean Code Template: return E:float [[E]] ireturn [[E]] freturn COMP3131/9102 Page 547 May 8, 2017

36 Code Template: Examples: Expression Statement: E; [[E]] pop if it has a value left on the stack 1; ---> pop 1 + 2; ---> pop f(1,2) ---> pop if the return type is not void a = 1; ---> no pop ; ---> no pop COMP3131/9102 Page 548 May 8, 2017

37 Code template: Compound Statememts Push the label marking the beginning of scope to scopestart Push the label marking the end of scope to scopeend... [[DL]] // no code; [[SL]] Pop the scopestart label Pop the scopeend label Code will be provided COMP3131/9102 Page 549 May 8, 2017

38 Global Variable Declarations Provided for you (but only for scalar variables) Generate.field declarations Geneate the class initialiser<clinit> You need to add the initialisations for arrays All initialisers for global variables are assumed to be constant expressions as in C, although this was not checked in Assignment 4. COMP3131/9102 Page 550 May 8, 2017

39 Local Variable Declarations Instance field index available in VC.ASTs.Decl.java Call frame.getnewindex() to allocate indices consecutively for formal parameters and local variables: For a function (treated as an instance method), 0 is allocated to this For main (a static method), 0 is allocated argv and 1 to the implicitly declared variable vc$ COMP3131/9102 Page 551 May 8, 2017

40 lvalues (store) v.s rvalues (load) Let visitsimplevar do nothing (because we do not know by looking at this node whether the variable is a lvalue or rvalue) Generate an appropriate load or store in visitassignexpr Consider l = r (store for l and load for r): COMP3131/9102 Page 552 May 8, 2017

41 Generating Jasmin Directives.limit locals.limit stack.var.line COMP3131/9102 Page 553 May 8, 2017

42 .limit locals XXX Generated at the end of processing a function XXX is the current value of frame.getnewindex() COMP3131/9102 Page 554 May 8, 2017

43 Syntax:.var.var var-index is name type-desc scopestart-label scopeend-label Generated when a var or formal para decl is processed var-index, name and type are extracted from the Decl node The scopestart and scopeend labels from scopestart and scopeend stacks (Slide 549) COMP3131/9102 Page 555 May 8, 2017

44 .line XXX Source line where the instructions between this.line and the next are translated from Optional (you should leave it at the very end) Maintain a current line Generate a.line if the next construct is from a different line COMP3131/9102 Page 556 May 8, 2017

45 .limit stack XXX XXX is the maximum depth of the operand stack Calculating the value by simulating the execution of the byte code generated incrementally Example: iconst_1 iconst_2 iadd iconst_1 iconst_2 iconst_3 iadd iadd astore_1... frame.push() frame.push() frame.pop() frame.push() frame.push() frame.push() frame.pop() frame.pop() frame.pop() COMP3131/9102 Page 557 May 8, 2017

46 Some Language Issues Java byte code requires that all variables be initialised all method be terminated by a return Both are not enforced in the VC language All test cases used for marking Assignment 5 will satisfy the 1st restriction, You can satisfy the 2nd restriction by either having approprite returns in your test programs or optionally forcing your VC compiler to always generate an appropriate return for each function COMP3131/9102 Page 558 May 8, 2017

47 Reading Chapter 7 of the on-line JVM Spec (compiling Java) 8.4 (Red Dragon) or of Purple Dragon (for short-circuit evaluations) Assignment 5 spec Next/Last class: Chris s lecture COMP3131/9102 Page 559 May 8, 2017